1852.] OF THE ROYAL INSTITUTION. 1738 
Hence the distinction of at least two species of heating rays 
emanating at the same time from the same Juminous source. 
From the neglect of this distinction much confusion has been kept 
up: and statements involving such confusion have been repeated 
from one elementary treatise to another. 
Again; notwithstanding that the experiments of Leslie and others 
on the absorption of heat from non-luminous sources, as well as those 
of Professor Bache on the radiation from surfaces, demonstrate that 
the effect lias no relation whatever to colour, yet the contrary asser- 
tion has been often persisted in. 
Again ‘ dark heat”’ is often spoken of without recollecting that 
rays of the very same quality and properties exist in the compound 
radiation from luminous sources. 
The conclusions drawn from later experiments, (performed with 
all the advantages derived from the beautiful invention of the thermo- 
electric instrument of Nobili,) in many instances, are still vague, 
from want of attention to the distinction of different species of heat 
emanating at the same time from the same source. 
Melloni, in a most extensive and valuable series of experiments, 
taking as the sources of heat successively flame, incandescent metal, 
boiling mercury, and boiling water, and applying in each instance 
a long series of substances as screens, estimated the proportion of 
rays out of 100 stopped, which was very different for each screen 
and each source: evincing wide differences in ‘‘ diathermaneity,” 
while rock salt alone was almost totally ‘‘ diathermanous” to rays 
from all sources alike. 
But we must still ask, what species of rays were those respectively 
stopped and transmitted? To take the per centage simply is ambi- 
guous; the body of rays is not homogeneous ; the property of trans- 
missibility should be viewed in combination with other properties 
of the specific rays, such as those evinced in their relations to the 
texture or colour of the absorbing surface. 
Nor is the ambiguity removed, though the difference of source 
is specially referred to, if the heterogeneity of rays from the same 
source be overlooked. The mere classification of sources into dumi- 
nous and non-luminous will not suffice: still less a reference to their 
temperatures : it being perfectly well known that the temperature of 
luminosity is very different for different substances.* 
Again Melloni has shewn that the diathermaneity is not pro- 
portional to transparency, by a classified series of transparent screens 
with the /amp. 
It must however be recollected that the term ‘ diathermaneity ” 
is applied indiscriminately to a heterogeneous body of rays: out 
of which some species of rays are entirely stopped, others entirely 
transmitted ; and the great differences in ‘‘ diathermaneity ”’ for heat 
* References in detail to all the different researches here mentioned, will be 
found in the Author’s Two Reports on the state of our knowledge of Radiant 
Heat in the British Association Reports, 1832 and 1840. 
